1. Field of the Invention
The present invention relates to a TDMA (time division multiple access) cellular mobile communication system, and more specifically to a handoff technique that is used for switching channels during communication.
2. Description of the Related Art
In a cellular mobile communication system, particularly in a digital system where the channel frequencies assigned to cell sites are considerably high and interference between adjacent cells occurs frequently even though the mobile unit remains stationary. Since the interference varies with different location of the mobile unit, the quality of the channel being in use constantly varies with the movement of the mobile unit. It is thus necessary to perform switching of channels even if the mobile unit does not cross the boundary between cells. This channel switching is known as "intra-cell handoff" in contrast with "inter-cell handoff" in which the channel switching takes place between adjacent cell sites when the mobile unit is moving across a cell border.
A known TDMA cellular system, as described in a paper "Mobile Communication Switching System for Voice/Nonvoice Services", IEEE GLOBE COM '91, pages 1485-1489, Ishino et al, employs a plurality of speech coder/decoder units at the mobile telephone switching office (MTSO) for lowering the bit rate of signals to increase the capacity of the system. A plurality of modulator/demodulator units are installed in the base station and connected via duplicated 64-kps circuits to the speech coder/decoder units in a one-to-one correspondence. The speech coder/decoder units are connected to terminals of a MTSO switch where switched connections are established with the public switched telephone network. Line-cost reduction is achieved by time-division multiplexing three speech signals at each end of the transmission link.
Since the speech coder/decoder units at the MTSO are associated in a one-to-one correspondence with the modulator/demodulator units at the base station, there is a need to provide switchover between speech coder/decoder units during handoff, in addition to switchover between modulator/demodulator units. Additionally, due to the inherent difficulty in precisely matching the switchover instant at the MTSO with the switchover instant at the base station, a three-way trunk circuit is provided to ensure against possible interruption of connection.